The present invention relates to the correction of distortion in electrical signal transducers, and particularly in loudspeakers intended for use in high fidelity sound reproduction systems.
In the field of high fidelity sound reproduction, the primary concern is to provide components which reproduce the original sounds as accurately as possible. To this end, considerable research and development have been conducted in the industry with the aim of improving the performance of each of the component parts of such a system. It is a truism that a sound reproducing system can be no better than the poorest component thereof and the "weakest link" in a sound reproducing system is generally recognized to be the loudspeakers.
One reason for this is that a loudspeaker must convert an electrical signal into physical movement of a structural part and achievement of a high accuracy energy conversion of this type has proven to be a complex problem, particularly since relatively high energy levels are involved. As a result even the best loudspeakers produce noticeable distortion, particularly at the high and low extremes of the audible frequency range.
Numerous approaches for reducing loudspeaker distortion have already been proposed. For example, it is now common practice to effect a certain degree of distortion reduction by employing two or more speakers each designed for optimum performance in a respective frequency range, and to interconnect the speakers by a cross-over network. Based on the principle that the frequency range in which a given speaker has a nominally "flat" response depends on the physical dimensions of the speaker, it is common practice to employ small speakers to reproduce the higher frequency bands, large speakers to reproduce the lower frequency bands, and mid-sized speakers to reproduce the mid-frequency bands. Such systems are, relatively speaking, of limited effectiveness and provide only a modest improvement in overall frequency response.
A second approach involves the use of a number of frequency filters each arranged to pass only a narrow band of frequencies and each associated with means for individually adjusting the relative amplitude of the output of its associated filter. This permits the frequency response of an amplifier connected to all of the filters to be shaped in a manner to compensate for the frequency response of the speakers. Such arrangements require a relatively large number of components and are, as a result, fairly expensive. Moreover, the amplitude of the output of each filter must be adjusted in a specific manner for a given speaker. The adjustments necessary to achieve optimum, or even near optimum, distortion correction for a particular speaker are usually beyond the ability and patience of an ordinary user of audio equipment, who does not possess the necessary technical expertise or the measurement equipment required for good adjustment.
Simple forms of arrangements of this type may employ only a single lowpass filter for influencing the lower portion of the frequency range and a single highpass filter for influencing the upper portion of the frequency range. While such a simple arrangement can be adjusted with relative ease, it cannot produce a substantial correction because a properly designed highpass or lowpass filter will have a substantially uniform response, or attenuation, over the entire range of frequencies which it is to pass, which does not correspond to the nonuniform response of a loudspeaker over the low frequency and high frequency ends of the range of frequencies which must be reproduced in a high quality system.
It has also been proposed to effect distortion correction by connecting an integrator or differentiator in feedback with a signal amplifier. Integrators and differentiators, while they may be constituted by circuits similar to those of highpass and lowpass filters, differ significantly therefrom with regard to the values of their circuit components because the response of an integrator or differentiator to the frequency range over which it is designed to operate is completely different from that of a lowpass filter or highpass filter. For example, while, as noted above, the frequency response of a filter will be substantially uniform over the range of frequencies which it is to pass and will drop off sharply at the edge or edges of that range so that within the range of frequencies to be passed, the output signal from a filter will be an accurate representation of the corresponding frequency components of the applied signal, an integrator or differentiator has a gradually sloping frequency response over the entire range of frequencies on which it is designed to act and will produce an output signal which varies with frequency in a manner differing significantly from the original input signal.
Examples of systems employing an integrator or differentiator in feedback are disclosed in U.S. Pat. Nos. 2,948,778 and 3,014,096, both issued to Warner W. Clements.
When an electrical signal is fed into a transducer, for example an electromagnetic loudspeaker, it produces a force on the movable member of the speaker which is proportional to the current flowing through the transducer. This current is, in turn, proportional to the voltage of the signal, from which it follows that the force applied to the movable part of the transducer is proportional to the voltage of the signal applied to the transducer.
In order for a transducer to perform ideally, with no distortion, it is known that the velocity imparted to the movable portion thereof must be proportional to the voltage of the applied signal, and hence must be proportional to the force acting on that movable portion. Establishment of this relationship with total accuracy requires that the only force opposing the motion of the movable portion be proportional to the velocity of the transducer and such a force is produced by the motional resistance of the air against which the movable portion moves. If this were the only force opposing the motion of the transducer, and if the movable portion of the transducer had sufficient structural integrity, as to approach that of an ideal inelastic solid, ideal performance would result.
However, it is generally known that this is not the case. There are two major additional forces which oppose ideal performance and are significant causes of distortion. First, since the movable part has some mass, it has an associated inertia which always acts to oppose any change in velocity. This inertial force is equal in magnitude and opposite in direction to the acceleration of the movable part. Because of this force it is presently desirable to try to minimize the mass of the moving part, and to a certain extent this is accomplished at the expense of introducing undesirable structural elasticity. Secondly, there is the force exerted on the movable part by its suspension and by air sealed within the speaker enclosure, which combine to create the stiffness of the support for the movable part. These produce a reaction, or restoring, force which acts in a direction to maintain the movable part in its neutral position and this reaction force is equal in magnitude and opposite in direction to the distance of the movable part from its neutral position.
Systems employing an integrator or differentiator are based on a recognition that the acceleration of the movable part is proportional to the time derivative of its velocity, while the excursion of the movable part is proportional to the time integral of its velocity.
While, in theory therefore, an integrator or differentiator connected in feedback with an amplifier should help significantly to compensate for one source or another of loudspeaker distortion, it has been found in practice that prior art arrangements of this type are of limited utility. One reason for this is that it is difficult to properly adjust such devices so as to obtain accurate feedback signals of appropriate amplitude for a given speaker. It has also been found that such a system functions properly only when the relative amplitude of the signal components provided by, or the gain of, the integrator or differentiator is kept small, so that the system will only provide satisfactory correction for small amounts of speaker distortion. In fact, further, if the gain of the integrator or differentiator of such a circuit is increased above a very low level, toward the value required to completely correct for one source of distortion in a speaker of average quality, it tends, because of the feedback connection, to generate instabilities which themselves measurably distort the resulting sound reproduction.
It is believed that one reason for these shortcomings is that such prior art systems are designed in dependence on the assumptions that the restoring force acting on the movable part of a speaker is linearly proportional to the excursion of that part from its neutral position and that the effective mass of the movable part of the speaker is constant, which assumptions are only first order approximations of the conditions found to exist in practice. In systems employing a differentiator or integrator in feedback, and designed according to the abovementioned assumptions, the differences between the approximations which those assumptions represent and the relationships which exist in practice themselves cause signal reproduction errors whose amplitudes bear a positive exponential relation to such differences, unless the gain of the integrator or differentiator is turned down to such a low level that the element no longer has a significant beneficial influence on the resulting sound reproduction.